Ubuntu Server 18.04LTS Can't extend default LV?
LVM creates metadata backup on every change. It can't back up the data to /etc/lvm/archive as / is full. Either delete some files and try again, or use the -A n switch to not back up this change.
Reference man lvm (emphasis mine):
-A|--autobackup {y|n}Whether or not to metadata should be backed up automatically after a change. You are strongly advised not to disable this! See vgcfgbackup(8).